The present invention relates to a method and apparatus for welding workpieces together and particularly relates to a method and apparatus for welding workpieces together to provide a structure having a plurality of areas with joined and bonded locations at least one of which areas are different in depth than the depth of one of its adjacent areas.
The design of certain products require that a plurality of workpieces be joined together to provide a particular structure. Dependent on the geometric configuration of such a structure, it is necessary that certain locations be joined and bonded at different depths. One such structure of the workpieces is a railway car door.
Railway car doors are constructed of various panels, panel members, angle members, door frame members, plates, gasket retainers, hat shape members, etc. In the past, these railway car door components have been joined together with rivets. Due to the geometric configuration of these components required by various operational and strengthening construction features and such other desirable features known to those in the art, the location of the areas of the parts to be joined are at different depths.
In the past, such joining has been performed by the use of rivets. A railway car door of the riveted construction has presented certain problems in its construction. One such problem is that when the parts are riveted together, a multiple assembly process must be followed which requires the perforation of the parts to be joined and the alignment of those perforations, which may necessitate reaming the perforations to assure alignment of the parts. Rivets are then positioned in the perforations and the rivets are then headed to assemble the parts together. This multiple step process of assembling the parts of a railway car door is indeed complicated, time consuming and, as a result, expensive.
The placement of rivets is important since gapping between two of the door members along their edge is created if the rivets are placed too close to each other or the members are not properly aligned. This effect is commonly referred to as "gapping" or "scalloping." In addition, a door constructed by utilizing the riveting process must of necessity have protruding heads which, in some cases, limits the design construction and clearance of the railway car door. Due to the newly enacted Federal Occupational Health and Safety Act requirements, riveting generally creates a high noise level in the assembly location and thereby does not provide a desirable environment for the assembly personnel. An additional problem associated with a riveted railway car door is that when such a door is damaged, an extensive number of steps must be followed to repair the car door parts and re-rivet the damaged parts together so that the door is again operational.
The present invention solves these prior art problems by the use of gas metal arc welding. The gas metal arc welding process maintains an electric arc between a continuously fed electrode welding wire and the workpieces. Shielding is necessary as in any welding process to exclude oxygen and nitrogen in the air from the arc and the metal, thus eliminating the formation of oxides and nitrides which decrease metal-weld ductility and sometimes strength. In the gas metal arc process, shielding is accomplished by the flow of a shielding gas from a welding head which protects and shields the welding zone. After the shielding gas begins to flow, the welding wire is fed toward the workpieces which are assembled in their respective positions in which they are to be joined. The welding power is supplied between the workpieces and the welding wire to thereby strike an arc therebetween. As the welding wire continues to move towards the workpieces, an opening is burned into the workpieces which is subsequently filled with molten metal melted by the arc. The shielding gases protect the melting electrode wire and the welded metal from the air. After the weld is allowed to cool, it is no longer necessary to provide the shielding gas in the welding zone.
One known machine for joining and securing metal parts together is disclosed in U.S. Pat. No. 2,466,668 to Weightman. Weightman discloses an electrical resistance or spot welding machine which resistance welds a plurality of elongated profiles along their length to form a beam. Another prior art resistance welding device is disclosed by U.S. Pat. No. 2,464,906 to Unger. Unger discloses a resistance welding machine for resistance welding corrugated stiffeners to side sheets. The resistance weld is made in the troughs of the corrugated stiffeners to join the corrugated stiffeners and side sheets together into a panel.
In the past, the resistance spot welding process to assemble railway car door parts has been tried, as shown in U.S. Pat. No. 1,240,634 to Williams but modern assembly of railway car door components utilize a riveting process. It should be noted that the spot welding process has inherent problems as described below.
One of the inherent problems in the resistance welding process is obtaining a high quality welded joint. In order to obtain a proper resistance welded joint, it is necessary that the surfaces of the material to be joined are clean and do not have any scale, dirt or plating thereon which affect the resistance welding process. Such a cleaning process inherently increases the number of steps and costs involved in resistance welding workpieces together. Even after the cleaning process, it is possible that some scale, dirt or plating can subsequently contaminate the area to be joined or some scale, dirt or plating may remain in the area to be joined. The parts must also be degreased before performing the resistance welding process.
Another problem associated with the automation of the resistance welding process is that when multiple resistance welding heads are brought into contact with the workpieces, each of the welding heads must weld simultaneously or the welding heads must be well insulated from each other. This inherent problem creates difficulty in designing a machine where the welding heads are selectively fired dependent on the position of the welding head with respect to the workpieces.
Another known apparatus for welding workpieces together includes a plurality of gas metal arc welding heads which are movable toward and away from the workpieces to be joined together. This apparatus of the prior art does not provide for welding workpieces together when the locations to be welded are at different depths or independent control of the welding heads for selective firing thereof.
The above-mentioned apparatus of the prior art for welding workpieces together includes a plurality of gas metal arc welding heads which are moved in unison toward and away from the workpieces to be joined together. This prior art apparatus is capable of welding workpieces together so long as the locations to be welded together are of the same depth.